Field of the Invention
This invention relates to thermoplastic adhesives for surface-mounting employing a combination of thermoplastic-polyimide (TPI) partially cured B-staged and fully cured C-Staged adhesives and in particular their advantageous use in joining materials of mismatched Coefficients of Thermal Expansion (CTE).
Description of the Prior Art
As an example, the lamination of semiconductor die to an aluminum heat sink is common in high-power applications, as the heat sink dissipates the heat generated from the semiconductor. As the laminated materials have severely mismatched CTEs, such as aluminum, approximately 23 ppm/° C. and semiconductor, approximately 2-8 ppm/° C., the bond line between the die and the heat sink undergoes significant inter-laminar stress during the wide temperature excursions of processing and use. Failure of the bond line between the semiconductor and the heat sink will dramatically reduce thermal dissipation between the surfaces, resulting in overheating and failure of the semiconductor.
There are a number of existing technologies employed for die-attach (chip-bonding or mounting) lamination of a semiconductor to an aluminum heat sink for thermal management.
Conventional die-attachment is generally done with epoxy that has been filled with metal powder to enhance thermal conductivity. Often, electrical conductivity of the die-attach bond line is also critical.
Thermoset epoxy polymers are brittle, both unfilled and especially filled, and so the die-attach epoxy bond line is designed to withstand the inter-laminar stress without inducing cracks in the epoxy which will propagate with time and temperature cycling. Reducing the thickness of the bond line exponentially increases the inter-laminar stress between the die and heat sink, and so epoxy bond lines have a minimum thickness of 1-1.5 mil (25-37 um) and are often considerably thicker.
To maximize thermal transfer and potentially electrical transfer between the die and heat sink, highly conductive metal powders such as silver are compounded into liquid, uncured, A-staged epoxy resin. The concentration of these metal powders can reach 80% by weight or more as solids in the cured bondline. As silver is a precious metal, and is often used in a costly micro- or even nano-sized format, the cost component of the metal in the bondline is significant, especially as the epoxy bondline needs to be 1 mil (25 um) thick or more.
When thermal conductivity, but not electrical conductivity, of the die-attach bondline is desired, ceramic powders are used as fillers in epoxy bondlines. Ceramic powders, such as alumina and boron nitride, are high thermal-conductivity dielectrics.
In processing, the die-attach epoxy is applied to the heat sink surface manually or with an automated dispenser. After the semiconductor die is precisely placed onto the epoxy surface, the subassembly's bondline is then cured with heat in a controlled manner that allows outgassing and avoids voiding. Some pressure may also be applied.
The invention disclosed herein, i.e., the use of a combined C-staged and B-staged TPI adhesive bondline has the following advantages in die-attach over the epoxy technology described above:
The TPI polymer will not crack, allowing much thinner bondlines between CTE mismatched surfaces and potentially enabling higher loadings of metal particles which, if used in an epoxy adhesive, would further embrittle the already brittle cured epoxy.
Thinner die-attach bondlines will enable higher thermal and electrical transfer rates between the die and the heat sink.
Thinner die-attach bondlines will utilize much less material, providing substantial cost savings.
While epoxy die-attach bondlines have a maximum temperature rating of 175° C. or less, TPI bondlines can operate continuously at well above 250° C. This will become increasingly important with the transition to wide band-gap semiconductors, such as SiC and GaN, which can operate very efficiently at high temperature.
Die-attachment can also be done with eutectic solders, in pre-forms or as paste, compounded with an organic flux that prevents oxidation of the surfaces at high temperature and promotes surface wet-out, ensuring an optimal bondline. Solder die-attach is highly electrically and thermally conductive and can provide a robust ductile bondline that provides a buffer between CTE mismatched surfaces.
High-performance solders are generally made with precious metals such as silver (Ag) and gold (Au) and, for die-attachment, require extreme reflow temperatures, such as 363° C. for gold-silicon alloy (AuSi). Precious-metal solders generally have bondline thicknesses in the 1-10 mil (25-250 um) range. As aluminum heat sinks do not provide a readily solderable surface, the targeted aluminum area requires a metal plating or braising of a precious metal to ensure a robust solder joint between the semiconductor die and the heat sink. This primer metallization is generally between 0.08 and 0.15 mil (2-4 um). Both the raw materials and required processes for eutectic solder die-attachment are very costly.
The invention disclosed herein, i.e., the use of a combined C-staged and B-staged TPI adhesive bondline has the following advantages in die-attach over precious-metal solder technology described above:
The material cost of TPI polymer is lower than precious-metal solder.
The equipment requirements and process cost of the TPI bondline is lower than the process cost of precious-metal solder, as much lower temperatures are utilized in curing TPI than in reflowing precious-metal solder.
TPI generally does not require a prime coat to bond to aluminum. Precious metal plating or braising of the aluminum surface to be bonded is expensive in both material and process cost. In the invention, when priming of a metal, ceramic or semiconductor surface is required to ensure a robust bondline, a simple wipe with A-staged TPI liquid and then a quick bake to drive off the solvent and B-stage the polymer suffices.
There is therefore potentially much higher thermal and electrical conductivity in a much thinner TPI bondline filled with metal particles.